OBJECTIVE To understand mechanisms of sociosexual development in male tamarins to complement our extensive knowledge of female tamarin development RESULTS Using urinary hormonal measurements and behavioral observations we have studied the same males in family groups, when paired and after they became fathers. Males in family groups had high levels of reproductive hormones which rose when paired with a new mate, but fell to family levels when these males became fathers. Males in family groups showed high rates of erection and mounting, but directed this sexual activity toward brothers and fathers rather than to females. Males received high levels of aggression from other group members, but mainly from siblings rather than adults. Additional studies have validated the use of a non-invasive measurement of scrotal width which can be used to monitor hormonal development. We have simultaneous longitudinal measurements of scrotal change and changes in reproductive hormones through development. We have completed a longitudinal study of male hormonal and sociosexual development and are comparing the nature of social interactions of eldes t brothers with their fathers. DISCUSSION We find males have very different development of both reproductive hormones and sexual behavior from females. We were surprised by high levels of homosexual behavior in family groups. FUTURE DIRECTIONS To understand role of high levels of sexual activity and hormonal function in subordinate adult males in cotton-top tamarins. FUNDING NIMH 35,215, MH 00177